The labelling of tissue with radioactive substances is a well established method in medical diagnosis and surgery. Employing radiation detecting devices the specific uptake of radioactivity, e.g. in malignant tissue, is detected and localized. Various types of gamma cameras provide scintigraphic images of the radiation distribution within a patient for diagnostic purposes. However, they are too bulky and too slow for radio guided surgery. Hand-held gamma probes with a head consisting of a radiation detector, shielding and a collimator are widely used for that purpose (e.g. WO 02/44755 A2). Such a probe is moved by the surgeon across a suspected region of the patient to locate e.g. cancerous tissue manifested by excess radiation as compared to the surrounding.
The activity injected into a patient should be kept as low as possible for the sake of the patient and the medical staff. This requires the detection efficiency of the gamma probe to be as large as possible. Assuming a point-like gamma source, the detection efficiency is predominantly determined by the solid angle covered by the gamma detector and by the detection efficiency. Large volume detectors cannot be used since the probe head needs to be light-weight and compact to allow for accurate manoeuvring and precise pointing at tumour manifestations. Therefore dense scintillators or solid state diodes composed of heavy elements are mandatory. More important, the distance of the detector to the target needs to be minimized. Assuming a typical distance of malignant tissue to the body surface of 10 mm, the detection efficiency of a probe placed directly at the body surface is four times respectively nine times larger than the efficiency achievable at 10 mm and 20 mm apart from the surface. Collimation of the field of view as discussed below often requires retracted position of the detector. These conditions lead to a generally accepted minimal efficiency corresponding to a sensitivity of about 5 cps/kBq and thus to a minimal required activity of a few kBq for a target to be detectable.
Unfortunately, gamma rays from the labelled site tend to scatter off the surrounding tissue and organs, thereby contributing to a diffuse radiation background rendering the localization of the original source difficult. In addition commonly employed radio pharmaceuticals enrich non-selectively in organs like liver or brain and are present throughout the whole body to some extent. Therefore intense background radioactivity is prevalent near tumour sites. One possibility to distinguish a scattered gamma ray from a direct gamma ray emitted by a source is by its energy. Only gamma rays coming in direct line from the source into the detector possess their full energy, whereas in any scattering process some energy is lost. Employing an energy sensitive detector and selecting events with the full gamma ray energy therefore allows to suppress background of scattered gamma rays. This method is only applicable with radio isotopes which emit gamma rays of one energy like 99mTc, while isotopes with several gamma transitions (e.g. 111In: 171 keV, 245 keV) are excluded. Moreover, not all gamma rays are fully absorbed in the detector. Depending on the type and size of scintillator or solid state diode used as detector, and on the energy of the gamma ray, the majority of gamma rays may deposit only part of their energy and will thus be assigned to background. Consequently the remaining rate of accepted direct gamma rays from a source may be severely lowered, reducing the sensitivity to detect and locate malignant tissue unless the statistical accuracy is re-established by an increased activity applied to the patient.
Another approach to suppress not only the background of scattered gamma rays but also the background radiation from other sources nearby the suspected tumour site is to utilize shielding and collimating material with the detector. Shield and collimator are made from dense, heavy metals like lead or tungsten alloys, yielding highest gamma ray absorption. The collimator is meant to define the field of view of the detector. For proper location particularly of small tumours high spatial selectivity is demanded, which requires a narrow opening cone of the collimator (compare H. Wengenmair et al.; Der Nuklearmediziner Nr. 4, 22 (1999) 271). The narrower the cone the smaller is the body volume contributing to the background signal, while the target signal from a tumour stays constant. At a distance of 30 cm (far field) the full width at half maximum (FWHM) of the radial signal rate distribution, determined by the cone and detector geometry, should be ≦40° for universal gamma probes. For cases with close-by background sources or unfavourable target to background ratio a narrower distribution is preferable. In typical applications the distance of the radiation sensitive part of the probe to the tissue to be inspected is about 10 mm. To distinguish close lying point-like sources, e.g. neighbouring lymph nodes, the spatial resolution of the probe at close distance (near field) to the target needs at least to be <25 mm. For the localization of very small lymph nodes for example prevalent in head and neck regions a near field precision of <15 mm FWHM is necessary. On the other hand a too narrow collimation is counter productive if large body regions need to be scanned during surgery. In that case a wider cone is preferable to shorten the screening time. Exchangeable and adjustable collimators (see e.g. U.S. Pat. No. 5,036,201, U.S. Pat. No. 4,959,547) are used with available gamma probes to adopt the cone characteristics to the application.
Background radiation may hit the probe from almost any direction. Therefore collimation and shielding needs to protect the detector from all sides but the opening cone. It was even suggested to use large shields external to the gamma probe (U.S. Pat. No. 5,148,040). In the worst case the injection depot of a radio pharmaceutical may be located in the vicinity of the probe with an activity 1000 times above the activity of cancerous tissue. Shield thickness of <4 mm is sufficient to provide an optimal absorption factor of >103 for background radiation of 140 keV energy emitted by 99mTc. Already for 364 keV gamma rays of the radio isotope 131I the attenuation of strongest absorbers, like tungsten, is no longer sufficient to reach this background suppression factor. To keep the weight of the probe well below 1 kg and the head diameter below 25 mm a maximal shield thickness of 9 mm is applicable, yielding a suppression factor of about 20. Increasing the shield thickness to 18 mm tungsten or 25 mm lead would reestablish the wanted background reduction. However, in that case the probe head would become too heavy and too bulky to be suitable.
For many medical applications PET radio pharmaceuticals like 18F-FDG turn out to be superior over conventional low energy tracers because of much higher specific concentration in malignant tissue, potentially leading to higher sensitivity and selectivity. However, for the 511 keV gamma rays of 18F-FDG the suppression factor of 9 mm thick tungsten is only 10 which severely limits the use of state-of-the-art probes for many applications.
As such, there is a need for a gamma probe, which may be employed in detecting and localizing gamma rays of 511 keV and even higher energies, which avoids the above mentioned restriction of contemporary art probes.